[CMake] Don't use comma as an alternate separator

[phosek]

Summary

Using comma can break in cases when we're passing flags that already
use comma as a separator.

Fixes PR35504.

[beanz]

LGTM.

llvm/trunk/cmake/modules/LLVMExternalProjectUtils.cmake

function(llvm_ExternalProject_Add name source_dir)
 
   # Find all variables that start with a prefix and propagate them through
   get_cmake_property(variableNames VARIABLES)
 
   list(APPEND ARG_PASSTHROUGH_PREFIXES ${nameCanon})
   foreach(prefix ${ARG_PASSTHROUGH_PREFIXES})
     foreach(variableName ${variableNames})
       if(variableName MATCHES "^${prefix}")
-        string(REPLACE ";" "," value "${${variableName}}")
+        string(REPLACE ";" "|" value "${${variableName}}")
         list(APPEND PASSTHROUGH_VARIABLES
           -D${variableName}=${value})
       endif()
     endforeach()
   endforeach()
 
   if(ARG_USE_TOOLCHAIN)
     if(CLANG_IN_TOOLCHAIN)
CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                ${ARG_CMAKE_ARGS}
                ${PASSTHROUGH_VARIABLES}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     BUILD_ALWAYS 1
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1
-    LIST_SEPARATOR ,
+    LIST_SEPARATOR |
     )
 
   if(ARG_USE_TOOLCHAIN)
     set(force_deps DEPENDS ${TOOLCHAIN_BINS})
   endif()
 
   llvm_ExternalProject_BuildCmd(run_clean clean ${BINARY_DIR})
   ExternalProject_Add_Step(${name} clean